Recently, LEDs have become very popular in general automotive lighting as a replacement for halogen low-voltage lighting technology. Design engineers are quickly recognizing the advantages of using LED lighting, which includes long operating life and low-operating DC voltages.
On the other hand, more complex and more expensive automotive electronics control systems are required to enable these emerging developments. New ultra-low-end 8-bit microcontrollers (MCUs) allow designers to significantly reduce costs and system complexity, bringing this alternative lighting solution to profitable levels.
The high-brightness LEDs (HBLEDs) continue to attract converts as new applications are enabled by low-end MCUs. These newly enabled HBLED uses include displays, automotive signage, signals, mobile appliances, and illumination.
In general, LEDs have a nonlinear I-V (current/voltage) behavior and thus current limitation is required to control two important issues, power dissipation and light output. Since both dissipation and output are current-dependent, the ideal source for driving an HBLED is a constant current source. High-brightness LEDs differ from standard LEDs because of their power output. Traditional LEDs are generally limited to less than 50 mW, while HBLEDs can provide 1 to 5W.
There are different approaches to get a constant-current source, such as using a series resistor, or a linear current source, or using a switching regulator. The most versatile and energy-efficient solution is the switched-mode approach. A switching regulator controls a current flow by chopping up the input voltage and controlling the average current by means of a duty cyclewhen a higher load current is required by the load, the percentage of on-time is increased to accommodate the change.
Switching regulators have four functional components: Power switch, rectifier, series inductor, and capacitor (seen here).
The current flowing through the HBLED is a consequence of the average energy stored in the inductor, L. When the switch is on, the inductor stores as much energy as possible, and the current flowing through the HBLED increases until it reaches the maximum limit or the switch time on has expired. Then, the switch is open and the energy stored in the inductor is providing current to the HBLED. This current is decreasing from the maximum level until the low level is reached or the switch time off time has expired (see below).
The duty cycle tailors current flowing through the HBLED.
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It is relative simple to achieve dimming in a MCU-based HBLED controller after current control has been solved. There are different techniques for dimming a HBLED, some of which may involve patent issues. In all cases dimming is achieved under the same basics, fully turning on the HBLED at its normal operating current for certain TOn amount of time and then completely turn it off for certain TOff duration. Doing so periodically at relative high speed (for the human eye to follow) will give the impression that the HBLED light intensity dimmed. The three most used techniques are pulse-width modulation (PWM), frequency modulation, and bit-angle modulation.
In the PWM modulation (see first figure below) the frequency should be fixed and the dimming level is a result of the duty cycle. Frequency modulation (middle figure) uses the concept of a fixed-width pulse but the frequency is variable. Dimming level is proportional to the number of fixed-width pulses in a defined period of time. Bit-angle modulation (bottom figure) is based on a binary pulse train that contains the intensity value. Every bit in the pulse train is stretched proportionally to its significance. If the least significant bit b0 has a duration of 1, then bit b1 has a duration of 2, bits b2 through b7 have durations of 4, 8, 16, 32, 64, and 128, respectively.